The present invention relates to a silica-alumina composite sol and a method for producing it. Particularly, it relates to a silica-alumina composite sol suitable for forming an ink-receiving layer of a recording medium for an ink jet printer, a method for producing it and a recording medium.
In recent years, reflecting wide use of digital cameras and computers, the hard copy technology to record images thereof on e.g. paper sheets has been rapidly developed. As hard copy recording systems, various systems have been known including not only a system wherein a display indicating an image is directly photographed by silver halide photography, but also a sublimation type thermal transfer system and an ink jet system.
Among these, an ink jet system is a system wherein ink droplets comprising dyes and a large amount of a solvent are ejected at a high speed from nozzles to a recording medium. An ink jet printer has been widely used in recent years, since full coloring and high-speed printing are easy, and the printing noise is low.
As the recording medium for an ink jet printer, in order to quickly absorb inks and to present a clear image, one having a substrate such as a paper sheet or a film, and a porous ink-receiving layer comprising inorganic fine particles of e.g. silica or alumina and a binder such as a polyvinyl alcohol, formed on the substrate has been known. The recording medium for an ink jet printer is required to absorb a large amount of the solvent contained in the ink, in the pores in the ink-receiving layer. Accordingly, the ink-receiving layer is required to have an appropriate pore size and a large pore volume. Further, the more transparent the ink-receiving layer is, the higher the color density will be, whereby a clearer image will be formed. Accordingly, one having a high transparency is preferred as the ink-receiving layer.
Further, since aqueous inks are employed for the ink jet recording system, it becomes important that even if exposed to water after formation of the image, the ink-receiving layer will have no defect in appearance, or the inks will not bleed due to flow of the dyes in the inks (hereinafter referred to as water resistance), quality of the recorded product will not deteriorate by scuffs generated when the surface of the recording medium is in contact with some sharp-edged stuff (hereinafter referred to as scuffing resistance), and the degree of glossiness on the surface is high (hereinafter referred to as glossiness), in addition to the above-mentioned ink absorptivity and transparency.
A silica type material such as a silica gel has pores in moderation. However, the surface of the particles of silica is usually negatively charged, and silica does not absorb an acid dye or a direct dye having anionic dissociative groups, to be used for ink jet, and silica has a low water resistance.
Accordingly, a method as disclosed in JP-A-60-257286, wherein polyaluminum chloride is contained in the ink-receiving layer, is mentioned. However, since the polyaluminum chloride is a water-soluble salt, the polyaluminum chloride in the ink-receiving layer itself dissolves in water, thus generating defects in appearance in a form of pits on the surface of the ink-receiving layer, and the water resistance is not necessarily adequate. Further, in a case of a long-term preservation, the polyaluminum chloride undergoes migration to clog the pores of the ink-receiving layer, thus decreasing the ink absorptivity.
Further, a method for producing a colloidal silica sol charged positively, by coating the surface of silica with alumina, is disclosed in JP-B-47-26959. The method comprises gradually adding a silica sol having particles sizes of from 2 to 150 nm to an aqueous solution of polyaluminum chloride, aging the mixture until the pH becomes constant, i.e. usually it becomes 4 or less, and then adding an alkali to increase the pH of the mixture to a level of from 4.5 to 7.0. By this method, a silica sol having its surface coated with alumina, having excellent transparency and stability, can be obtained. However, since secondary aggregate particles are not formed, a xerogel obtainable by drying the silica sol has a small pore volume and pore radius, whereby the ink-receiving layer formed by employing the silica sol may have an inadequate ink absorptivity, in some cases.
An ink-receiving layer formed by using an alumina hydrate such as pseudo-boehmite, is excellent from the viewpoints of ink absorptivity, transparency, water resistance and glossiness, but has a problem in view of scuffing resistance. This is estimated to be because the alumina hydrate is not spherical. To overcome the problem, a method as disclosed in JP-A-7-76162, wherein a silica gel layer having a thickness of from 0.1 to 30 xcexcm is provided on a porous layer made of pseudo-boehmite, is mentioned. However, this method has such a drawback that the silica gel layer inhibits the ink absorptivity, and is disadvantageous in view of industrial production because of the two-layer structure.
The present invention provides a silica-alumina composite sol which is a colloidal solution having aggregate particles containing silica and alumina dispersed in an aqueous medium, wherein the silica is such that the primary particles are spherical and the average particle size of the primary particles is from 2 to 200 nm, the average particle size of the aggregate particles is at least twice the average particle size of the silica primary particles and at most 1,000 nm, the xcex6-potential of the aggregate particles is at least +10 mV, and the pH of the solution is from 3 to 9.
The present invention provides, as a preferred method for producing the above-mentioned silica-alumina composite sol, the following first production method and second production method.
The first production method is a method for producing a silica-alumina composite sol, which comprises gradually adding an aluminum salt of which the liquid exhibits acidity when dissolved in water, to a silica sol containing such silica particles that the primary particles are spherical and the average particle size of the primary particles is from 2 to 200 nm.
The second production method is a method for producing a silica-alumina composite sol, which comprises mixing a silica sol with an alumina sol having a specific surface area of a xerogel obtainable by drying of at least 150 m2/g, to form aggregate particles containing silica and alumina, and adjusting the aggregate particles to have an average particle size of from 30 to 1,000 nm, by peptization treatment.
The present invention further provides a recording medium having a porous layer formed by coating and drying on a substrate a silica-alumina composite sol which is a colloidal solution having aggregate particles containing silica and alumina dispersed in an aqueous medium, wherein the silica is such that the primary particles are spherical and the average particle size of the primary particles is from 2 to 200 nm, the average particle size of the aggregate particles is at least twice the average particle size of the silica primary particles and at most 1,000 nm, the xcex6-potential of the aggregate particles is at least +10 mV, and the pH of the solution is from 3 to 9.
The silica-alumina composite sol of the present invention comprises aggregate particles containing silica and alumina, as colloidal particles, dispersed in an aqueous medium.
The silica in the aggregate particles are such that the primary particles are spherical and the average particle size of the primary particles is from 2 to 200 nm.
The silica-alumina composite sol of the present invention has silica primary particles being spherical, whereby a high scuffing resistance can be obtained when the coating layer is formed on the substrate.
If the average particle size of the silica primary particles is smaller than 2 nm, when the composite sol is dried, a xerogel having a large average pore radius and pore volume can not obtainable, such being unfavorable. If the average particle size of the silica primary particles exceeds 200 nm, when the composite sol is dried, a xerogel having a large specific surface area can not be obtainable, and a xerogel having a high property for fixing dyes can not be obtainable, such being unfavorable. The more preferred range of the average particle size of the silica primary particles is from 5 to 100 nm. The average particle size of the silica primary particles is measured by a transmission electron microscope.
The average particle size of the aggregate particles is required to be at least twice the average particle size of the silica primary particles. A conventional silica sol is produced not to contain aggregate particles so that a good stability and dispersibility can be obtained. However, in the silica-alumina composite sol of the present invention, the aggregate particles are positively formed. Since the composite sol of the present invention contains such aggregate particles, the pore volume and the average pore radius of the xerogel can be made large, whereby an ink-receiving layer having an excellent ink absorptivity can be formed.
The average particle size of the aggregate particles is required to be at most 1,000 nm. If the average particle size of the aggregate particles exceeds 1,000 nm, when converted to the xerogel, not only the transparency decrease, and the haze of the ink-receiving layer increases, but also the color density of a cyan dye will decrease in the water resistance test as mentioned hereinafter. The average particle size of the aggregate particles is preferably at most 500 nm, since better transparency can be obtained. The average particle size of the aggregate particles is preferably at least 30 nm.
The silica-alumina composite sol of the present invention has a pH of from 3 to 9. If the pH is higher than 9, the xcex6-potential of the aggregate particles will be low, such being unsuitable. On the contrary, if the pH is smaller than 3, alumina may dissolve, such being unsuitable. The more preferred range of the pH is from 3 to 7.
The silica-alumina composite sol of the present invention has a xcex6-potential of the aggregate particles of at least +10 mV. The surface of a conventional silica sol is negatively charged, whereby it has no property for fixing anionic dyes to be used for an ink jet printer. For example, in the water resistance test as mentioned hereinafter, the color density of the cyan ink is substantially 0. On the contrary, the silica-alumina composite sol obtained in the present invention has a positive surface charge, whereby it has a property for fixing colorants, and the color density of the cyan ink is at least 1.0 in the water resistance test as mentioned hereinafter. The more preferred range of the xcex6-potential of the aggregate particles is from +30 to +90 mV.
In the silica-alumina composite sol of the present invention, the xcex6-potential of the aggregate particles tends to be high with increase in the amount of alumina relative to the silica. The alumina is required to be added so that the xcex6-potential of the aggregate particles is at least +10 mV. The larger the specific surface area of the silica sol as the starting material, the more the alumina is required to be added, and the alumina is added preferably in an amount of at least 1 g as Al2O3 based on 100 g of the SiO2 component in the silica sol. Although there will be no problem even if an excessive amount of the alumina is added, the operation to remove impurity ions by e.g. ultrafiltration as mentioned hereinafter may not easily be carried out, such being disadvantageous. The addition amount of the alumina relative to the silica is preferably at most 900 g as Al2O3 based on 100 g of the SiO2 component in the silica sol. The more preferred range is from 5 to 400 g as Al2O3 based on 100 g of the SiO2 component in the silica sol.
In the composite sol of the present invention, the amount of impurities (elements except Si, Al, O and H) is preferably at most 10 mol % relative to the total amount of number of atoms of Si and Al, based on the number of atoms. In a case where an ink-receiving layer is formed by using a silica-alumina composite sol having a larger amount of impurity elements than this range, after formation of the image, the color of the dyes may change, or the surface of the ink-receiving layer will have defects in appearance in the form of pits in the water resistance test as mentioned hereinafter, such being unfavorable.
By removing the solvent from the silica-alumina composite sol of the present invention, a xerogel having a specific surface area of at least 50 m2/g, an average pore radius of at least 5 nm and a total pore volume having pore radii of from 1 to 100 nm of at least 0.35 cm2/g, can be obtained. The average pore radius is more preferably at least 10 nm. Further, the total pore volume having pore radii of from 1 to 100 nm is more preferably at least 0.50 cm2/g. These pore characteristics are measured by nitrogen adsorption/desorption method. Here, the average pore radius is represented by the value obtained by calculation from 2V/Axc3x97103 (nm), where V (cm3/g) is the total pore volume having pore radii of from 1 to 100 nm, and A (m2/g) is the specific surface area.
Of the xerogel obtainable by removing the solvent from the composite sol, in a case where the average pore radius is less than 5 nm, in a case where the total pore volume having pore radii of from 1 to 100 nm is less than 0.35 cm2/g, or in a case where the specific surface area is less than 50 m2/g, respectively, the ink absorptivity may be inadequate when the ink-receiving layer is formed, such being unfavorable. The more preferred pore characteristics of the xerogel obtainable by removing the solvent from the composite sol are such that the specific surface area is at least 100 m2/g, the average pore radius is at least 5.5 nm, and the total pore volume having pore radii of from 1 to 100 nm is at least 0.4 cm2/g. In the present specification, simply the pore volume is meant to be the total pore volume having pore radii of from 1 to 100 nm.
When the silica-alumina composite sol of the present invention is dried to remove the solvent, a xerogel having a good transparency can be obtained. The xerogel presents an excellent property for fixing dyes, transparency and glossiness, when used for the ink-receiving layer of the recording medium for an ink jet printer.
The property for fixing dyes, transparency and glossiness of the ink-receiving layer can be evaluated as follows. The silica-alumina composite sol is mixed with a polyvinyl alcohol in an amount of 1 part by weight per parts by weight of the solid content of the silica-alumina composite sol, followed by concentration or dilution as the case requires, to prepare a coating fluid having a total solid content concentration of 10 wt %. This coating fluid is coated on a white polyethylene terephthalate film (opaque film having a white pigment dispersed in the inside) so that the coating amount after drying is from 4.5 to 5.5 g/m2, followed by drying, to form a porous ink-receiving layer. Here, in a case of evaluating the transparency, a transparent polyethylene terephthalate film is employed as the substrate.
The water resistance (property for fixing dyes) of the ink-receiving layer is measured in such a manner that a film having a coating layer formed thereon is cut into an appropriate size, and soaked in a cyan ink (e.g. a color ink cartridge MJIC2C for an ink jet printer MJ-5000C manufactured by Seiko Epson Co., Ltd.) for 2 minutes, followed by washing with running water for 2 minutes to remove unfixed ink, and the color density of the ink-receiving layer by fixed cyan ink is measured by using a reflective color densitometer (RD-918, tradename, manufactured by Macbeth). To measure the reflected color density, a white reflection standard plate is put on the back side of the sheet for measurement. In this measurement, the color density of the ink-receiving layer is preferably at least 1, and in this case, the ink-receiving layer has an adequate water resistance.
The ink-receiving layer has a haze value of preferably at most 10%. The haze value of the ink-receiving layer is represented by the difference in the haze value between the polyethylene terephthalate film having the ink-receiving layer formed thereon and an uncoated polyethylene terephthalate film.
The glossiness of the ink-receiving layer is evaluated based on 600 specular glossiness as defined in JIS Z8741. The specular glossiness of the ink-receiving layer is preferably at least 20%.
Now, the first production method of the silica-alumina composite sol of the present invention will be explained more specifically. In the first production method, an aluminum salt of which the liquid exhibits acidity when dissolved in water, is added to a silica sol containing such silica particles that the primary particles are spherical and have an average particle size of from 2 to 200 nm.
The pH and the solvent of the silica sol as a starting material are not particularly limited, and the solvent is preferably water in view of easiness in the operation. For example, a commercially available silica sol such as Cataloid SI-40, tradename, manufactured by Catalysts and Chemicals Industries Co., Ltd., or Silicadol 20GA, tradename, manufactured by Nippon Chemical Industrial Co., Ltd., is preferably used. Further, the silica sol may optionally be diluted with water.
As the aluminum salt of which the liquid exhibits acidity when dissolved in water, preferred is a salt of aluminum hydroxide and a strong acid. In the present specification, the aluminum salt of which the liquid exhibits acidity when dissolved in water, will be referred to simply as an acid aluminum salt.
As the acid aluminum salt, an inorganic acid salt such as aluminum chloride, aluminum sulfate or aluminum nitrate, or an organic acid salt such as aluminum acetate, is preferably used. It is preferred that the acid aluminum salt is optionally dissolved in water, and mixed with the silica sol.
As the acid aluminum salt, particularly preferred is a polyaluminum chloride. The polyaluminum chloride is a compound having a chemical formula represented by [Al2(OH)nCl6xe2x88x92n]m (1 less than n less than 6, m less than 10). For example, a commercially available one such as Takibine #1500 or PAC250A, tradenames, manufactured by Taki Chemical Co., Ltd., may be mentioned. The polyaluminum chloride has a basicity of preferably at least 20%. If the basicity is less than 20%, the content of Cl is high relative to Al, such being disadvantageous in the case of removing impurity elements by e.g. ultrafiltration as mentioned hereinafter. The basicity is represented by (n/6) in the above-mentioned formula by percentage, and the specific method of measurement is defined by JIS K1475.
The method for mixing the silica sol and the acid aluminum salt is not particularly limited, and preferred is a method of adding the acid aluminum salt to the silica sol. It is preferred that a predetermined amount of the acid aluminum salt is gradually added to the silica sol as the starting material. When the acid aluminum salt is gradually added to the silica sol, alumina gradually forms and adheres to the surface of the silica particles in the sol. With the increase in the amount of adhered alumina, the surface potential of the sol particles shifts from negative to positive. During this shift, the surface goes through the state of the potential being 0, whereby aggregation of the particles takes place, and the aggregate particles containing silica and alumina are formed. It is preferred to stir the silica sol when the acid aluminum salt is added, to prevent the concentration of the acid aluminum salt being locally high.
On the other hand, if the silica sol as the starting material is gradually added to a solution of the acid aluminum salt, although a sol containing composite particles having alumina adhered to the surface of the silica sol particles is formed, the aggregate particles are less likely to form. Accordingly, a xerogel obtainable by drying the sol may be one having a small pore volume and average pore radius.
The temperature at the time of mixing the silica sol and the acid aluminum salt is preferably from 25 to 150xc2x0 C. If the temperature is lower than 25xc2x0 C., the reaction rate tends to be low, and alumina may not adequately adhere to the surface of the silica particles, such being unfavorable. A temperature higher than 150xc2x0 C. is unfavorable since the operation tends to be difficult.
As the addition amount of the acid aluminum salt, an amount enough to obtain a xcex6-potential of the particles of at least +10 mV, is required. The larger the specific surface area of the sol particles of the silica sol as the starting material, the more the acid aluminum salt is required to be added. In the present specification, in the case of the silica sol to be used as the starting material having an average particle size of the primary particles of from 2 to 200 nm, it is preferred to add the acid aluminum salt in an amount of at least 1 g as calculated as Al2O3, based on 100 g of the silica as calculated as SiO2. Although there will be no problem even if an excessive amount of the acid aluminum salt is added, the operation to remove impurity elements by e.g. ultrafiltration as mentioned hereinafter may not easily be carried out, such being disadvantageous.
In the first production method, it is referred to age a solution obtained by mixing the acid aluminum salt with the silica sol, at a pH of from 7 to 10 for aggregation, followed by peptization treatment, since a xerogel having a large pore volume can be obtained. In this case, by removing the solvent from the silica-alumina composite sol, a xerogel having a specific surface area of at least 50 m2/g, an average pore radius of at least 10 nm and a total pore volume having a pore radii of from 1 to 100 nm of at least 0.50 cm2/g, can be obtained.
The pH for the aggregation treatment is preferably from pH7 to 10. If the pH is lower than 7, no adequate aggregation may take place, and the pore radius, the total pore volume and the specific surface area can not be made large, such being unsuitable. On the other hand, if the pH is higher than 10, the color density of the ink in the water resistance test may decrease, such being unsuitable. The more preferred range of the pH is from pH7 to 9.
As the method to adjust the pH for the aggregation treatment to be from 7 to 10, a method of adding an alkali metal hydroxide or an alkali metal aluminate to a mixed solution of the silica sol and the acid aluminum salt, may be mentioned.
The aging is preferably carried out with stirring at a temperature of from 50 to 150xc2x0 C. for at least 1 hour. If the temperature for aging is lower than 50xc2x0 C., no adequate aggregation may take place, and the pore radius, the total pore volume and the specific surface area can not be made large, such being unsuitable. On the other hand, if the temperature is higher than 150xc2x0 C., the operation tends to be difficult, such being unfavorable. The longer the aging time, the more adequately the aggregation proceeds, and the larger the pore radius, the total pore volume and the specific surface area can be made, and an ink-receiving layer employing such composite sol will have an excellent ink absorptivity. However, if the aging time is too long, transparency will decrease, and accordingly adjustment is required in moderation.
In a case where the solution after the aggregation treatment contains a large amount of impurity ions such as alkali metal ions, it is preferred to remove the impurity ions for purification, prier to the successive peptization treatment. As the method for removing the impurity ions, it is preferred to use an ultrafiltration membrane, since the efficiency is high.
The above-mentioned aggregation treatment is effective also when applied to a mixture obtained by gradually adding the silica sol to a solution of the aluminum salt of which the liquid exhibits acidity when dissolved in water. Namely, it is possible to produce a silica-alumina composite sol with which a xerogel having a large pore volume and a large pore radius can be obtainable, by aging a mixture obtained by gradually adding the silica sol to a solution of the aluminum salt of which the liquid exhibits acidity when dissolved in water, at a pH of from 7 to 10 for aggregation treatment, followed by peptization treatment.
In the first production method, the aggregate particles can be formed more effectively by adding an electrolyte other than the acid aluminum salt to the silica sol. Here, the electrolyte to be added is not particularly limited so long as it has an aggregation effect to the silica sol or the alumina sol, and sodium chloride, calcium chloride, sodium sulfate, potassium acetate or magnesium nitrate may, for example, be mentioned. They may be used alone or in combination as a mixture. The addition amount is preferably from 1 to 70 wt % based on the weight of the silica (calculated as SiO2) in the silica sol as the starting material.
The method of adding the electrolyte is not particularly limited, and such an electrolyte may preliminarily be added to the silica sol, or it may be added to the acid aluminum salt followed by addition to the silica sol. Otherwise, the electrolyte may be added to a mixed solution having the acid aluminum salt added to the silica sol.
Then, it is preferred to remove the impurity ions such as unreacted acid aluminum salt or the added electrolyte, from the mixed solution having the acid aluminum salt added to the silica sol. As the method of removal, ultrafiltration is preferred.
Now, the second production method of the silica-alumina composite sol of the present invention will be explained more specifically. The second production method is a method for producing a silica-alumina composite sol, which comprises mixing the silica sol and the alumina sol having a specific surface area of a xerogel obtainable by drying of at least 150 m2/g, to form the aggregate particles containing silica and alumina, and adjusting the aggregate particles to have an average particle size of from 30 to 1,000 nm, by peptization treatment.
Specifically, it is preferred to employ the following method. Namely, a silica sol having an average particle size of the primary particles of from 10 to 200 nm and an alumina sol are mixed and reacted. Aggregation will take place by mixing. Then, the composite sol is adjusted to have particle sizes of the aggregate particles of from 30 to 1,000 nm, by peptization treatment.
The silica sol to be used in the present invention is preferably one having spherical particles as primary particles having an average particle size of from 10 to 200 nm. By using the spherical particles, the composite sol with the alumina sol will have the scuffing resistance. If the average particle size of the primary particles is at most 10 nm, the primary particles are too small, whereby a composite sol having a large pore radius and pore volume may not be obtained. Further, if it exceeds 200 nm, the specific surface area tends to be small, and one having a high color density of the cyan dye in the water resistance test may not be obtained.
The alumina sol to be used in the present invention is a sol comprising alumina hydrate as the sol particles, and when the alumina sol alone is dried to obtain a xerogel, the xerogel has a specific surface area of preferably at least 150 m2/g, more preferably at least 230 m2/g. As the alumina hydrate, boehmite (Al2O3.nH2O, n is from 1 to 1.5) is preferred. In the present invention, the specific surface area of the xerogel obtainable by drying the composite sol, is made large, by using such an alumina sol capable of forming a xerogel having a large specific surface area. As the specific surface area is large, the adsorption sites for dyes can be increased when the composite sol is dried, and an ink-receiving layer having a high color density of the cyan dye in the water resistance test can be formed.
As the silica sol as the starting material, spherical particles having primary particle sizes of from 10 to 200 nm are preferred, and a commercially available silica sol such as Cataloid SI-45P, tradename, manufactured by Catalysts and Chemicals Industries Co., Ltd. or Silicadol 2OGA, tradename, manufactured by Nippon Chemical Industrial Co., Ltd. may be preferably used. The pH, the solvent, etc. are not particularly limited. However, the solvent is preferably water in view of easiness in operation. Further, it may optionally be diluted with water.
The alumina sol as the starting material is a sol comprising an alumina hydrate as the sol particles. The production method is not particularly limited, and it can be obtained by a method of optionally aging an alumina gel obtained by hydrolysis of an aluminum alkoxide or by neutralization or ion exchange of an alkali metal aluminate or an aluminum salt, followed by washing and peptization.
Here, with respect to the alumina sol, the larger the specific surface area of the xerogel obtainable by drying the sol, the larger the specific surface area of the xerogel obtainable by drying the silica-alumina composite sol, and an ink-receiving layer having a high color density of the cyan dye in the water resistance test can be formed, such being favorable. The specific surface area of the xerogel obtainable by drying the alumina sol is preferably at least 150 m2/g, more preferably at least 230 m2/g. Alumina hydrate particles having such a high specific surface area, can be obtained, by controlling the conditions for aging the above-mentioned alumina gel obtained by hydrolysis of an aluminum alkoxide or by neutralization or ion exchange of an alkali metal aluminate or an aluminum salt, i.e. the pH, the temperature and the time.
It is preferred that the alumina gel obtained as mentioned above is optionally washed followed by peptization to obtain an alumina sol. The method of peptization is not particularly limited, and a method of adding an acid such as hydrochloric acid, nitric acid, acetic acid or amidosulfuric acid as a peptizer, or a method of peptization by mechanical means such as ultrasonic dispersion, may be mentioned. Further, they may be used together. The smaller the average particle size of the sol particles, the more uniform silica alumina composite sol can be obtained, and accordingly, the average particle size of the sol particles is preferably at most 500 nm. The average particle size of the sol particles is more preferably at most 300 nm.
In the second production method, the method of mixing the silica sol and the alumina sol is not particularly limited. The alumina sol may be added to the silica sol with stirring, or the silica sol may be added to the alumina sol with stirring. The temperature at the time of mixing is not particularly limited, and the mixing may be carried out under ordinary temperature or under optionally heating. However, if the temperature is too high, the operation tends to be difficult, and accordingly it is preferably at most 150xc2x0 C.
The addition amount of the alumina sol to the silica sol is preferably such that the alumina solid content is from 10 to 400 g based on 100 g of the silica solid content (calculated as SiO2). The larger the addition amount of the alumina sol, the higher the xcex6-potential of the composite sol tends to be high. As the addition amount of the alumina sol, it is preferred to add the alumina sol in an amount enough to obtain positively charged aggregate particles. In the case of using a silica sol having primary particle sizes of from 10 to 200 nm, it is necessary to add the alumina sol in an amount of at least 10 g as the alumina solid content based on 100 g of the silica solid content (calculated as SiO2). On the other hand, if the addition amount of the alumina sol is too large, when an ink-receiving layer is formed by using the obtained silica-alumina composite sol, the scuffing resistance of the ink-receiving layer may decrease, such being unfavorable.
In the present invention, the mixture of the above-mentioned silica sol and alumina sol is adjusted to have aggregate particle sizes of from 30 to 1,000 nm by peptization treatment. The method of peptization treatment is not particularly limited, and a method of adding a peptizer or a mechanical method such as ultrasonic dispersion may be mentioned. Further, they may be used together. As the peptizer, e.g. hydrochloric acid, nitric acid, sulfuric acid, acetic acid or amidosulfuric acid may be preferably used. They may be used alone or in combination as a mixture.
With respect to the silica-alumina composite sol synthesized by the first production method or the second production method, it may be directly used in the case where the average particle size of the aggregate particles is at most 1,000 nm, or as the case requires, the average particle size of the aggregate particles may be adjusted. The average particle size of the aggregate particles can be made small by e.g. ultrasonic dispersion. Further, peptization may be carried out by e.g. adding a peptizer. The peptizer is not particularly limited, and an inorganic acid such as hydrochloric acid, nitric acid, sulfuric acid or amidosulfuric acid, or an organic acid such as acetic acid, may be preferably used. Such a peptizer may be used alone or in combination as a mixture.
When the silica-alumina composite sol of the present invention is dried to remove the solvent, a xerogel having a high transparency and a high absorptivity can be obtained. Accordingly, by coating and drying on a substrate a coating fluid obtained by mixing the silica-alumina composite sol of the present invention with an optional binder, a recording medium having an ink-receiving layer which is excellent in ink absorptivity, transparency, water resistance, scuffing resistance and glossiness, can be obtained. The silica-alumina composite sol of the present invention may be filled in the paper substrate.
In the case of forming the ink-receiving layer from the silica-alumina composite sol, the binder is not particularly limited, and starch or its modified product, a polyvinyl alcohol or its modified product, a cellulose derivative such as carboxymethyl cellulose, SBR latex, NBR latex or polyvinyl pyrolidone may, for example, be mentioned.
The substrate for the ink-receiving layer is not particularly limited, and a film of a resin such as polyethylene terephthalate, a paper sheet such as woodfree paper or synthetic paper, a cloth, glass, a metal, leather, wood or ceramic such as pottery may, for example, be mentioned. Further, it may be formed on the top or the bottom of an ink-receiving layer formed by containing e.g. boehmite, a silica gel or a cationic resin other than one of the present invention.